This invention is generally directed to semi-suspension polymerization processes, and more specifically to processes for the preparation of small polymeric particles, for example, in one embodiment with an average diameter of from about 0.1 micron to about 5 microns. More specifically, the present invention is directed to semi-suspension polymerization processes for the preparation of economically submicron polymeric particles, useful as powder coatings such as carrier powder coatings, photoreceptor cleaning additives, and in some instances toner resins, wherein a mixture of monomer or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component are bulk polymerized until partial polymeriztion is accomplished, for example. In one embodiment, from about 10 to about 50 percent of monomer or comonomers are converted to polymer, thereafter the resulting partially polymerized monomer or comonomers is dispersed in water containing a stabilizing component with, for example, a high shear mixer, then the resulting suspension is transferred to a reactor and completely polymerized, followed preferably by washing and drying the polymer product. As indicated herein, the polymers obtained with the process of the present invention can, for example, be selected as coatings for carrier components utilized in developer mixtures comprised of toner and carrier compositions.
The formation of small polymeric particles for use in powder coatings and toner additives has been generally accomplished by emulsion polymerization and dispersion polymerization processes wherein a polymer in a liquid is heated to above its melting temperature and then cooled in the liquid to form particles. With these processes, it has been difficult to achieve low cost, clean (that is for example with no, or substantially no, impurities on the surface of the resulting particles), and/or dry particles of small particle size. The particles formed by suspension polymerization are generally larger than 50 microns thus they are not desirable as powder coatings or as toner additives. Grinding or attrition, especially fluid energy milling, of large particles to the size needed for powder coating, that is for example from about 0.1 to about 5 microns, is often not desirable both from an economic and functional viewpoint. Powder coatings with submicron particles are desirable as it allows complete coating of the particle, such as carrier beads, with a thinner film than is possible if only large particles are used. Also, submicron particles are very difficult to obtain by grinding or attrition. Further, processes such as spray drying of polymers suspended in solvent can result in polymer particles with particle sizes larger than one micron and a wide size range as well as trapping of solvent which interferes with the use of particles for coatings. Moreover, the solvent recovery in these processes is very costly.
There is disclosed in U.S. Pat. No. 3,505,434 a process wherein particles for fluidized bed powder coating are prepared by dispersing the polymer in a liquid which is heated to above the polymer melting point and stirred causing the polymer particles to form. The particles are then cooled below their melting point and recovered. However, this process does not, for example, enable particles with a size of below 50 microns.
Also, the suspension polymerization of monomer is known for the formation of polymer particles generally in a size range of about 200 microns and higher. The main advantage of suspension polymerization is that the product may easily be recovered, therefore, such a process is considered economical. However, it is very difficult by suspension polymerization to prepare very small particles as the monomer droplets tend to coalesce during the polymerization process, especially in the initial stage of polymerization where the droplets are very sticky. For example, there is disclosed in U.S. Pat. No. 3,243,419 a method of suspension polymerization wherein a suspending agent is generated during the suspension polymerization to aid in the coalescence of the particles. Also disclosed in U.S. Pat. No. 4,071,670 is a method of suspension polymerization wherein the monomer initiator mixture is dispersed in water containing stabilizer by a high shear homogenizer, followed by polymerization of suspended monomer droplets.
As a result of a patentability search there were located U.S. Pat. No. 4,486,559, which discloses the incorporation of a prepolymer into a monomer toner mix followed by emulsion polymerization; 4,680,200 and 4,702,988, which illustrate emulsion polymerization.
There thus remains a need for a process of obtaining very small polymeric particles, and more specifically submicron polymeric particles. Further, there is a need for semisuspension polymerization processes for obtaining clean and dry small, for example, from about 0.1 to about 5 microns in average diameter as determined by a scanning electron microscope, polymeric particles. Further, there is a need for semi-suspension polymerization processes that permit low cost, clean, and dry submicron polymeric particles that can be selected as carrier powder coatings, and as toner additives for photoreceptor cleaning.